1. Field of the Invention
The present invention relates to an image forming apparatus which includes a belt that rotates while carrying a toner image. In particular, the present invention relates to an image forming apparatus that can stably rotate a belt.
2. Description of the Related Art
A conventional image forming apparatus which includes a belt that rotates while carrying a toner image is illustrated in FIG. 3. Recently, there are plural-color and full-color image forming apparatuses using an electrophotographic system in which a plurality of photosensitive drums is arranged in a single row for respective colors. Moreover, there is an in-line type image forming apparatus in which a toner image of each color that is formed on each photosensitive member drum is superimposed in order on an intermediate transfer belt to form a color image.
FIG. 3 illustrates a schematic view of an example of a full-color image forming apparatus (or a full-color printer) using a conventional electrophotographic system of an in-line type, which includes an intermediate transfer belt (or an intermediate transfer unit). Moreover, the image forming apparatus includes four image forming units (or image forming stations), i.e., an image forming unit 1Y for forming a yellow-color image, an image forming unit 1M for forming a magenta-color image, an image forming unit 1C for forming a cyan-color image, and an image forming unit 1Bk for forming a black-color image. The four image forming units are arranged in a single row at constant intervals.
Electrophotographic photosensitive members of a drum type (hereinafter referred to as photosensitive drums) 2a, 2b, 2c and 2d are disposed in the image forming units 1Y, 1M, 1C and 1Bk, respectively, as image carriers. Primary chargers 3a, 3b, 3c and 3d, development devices 4a, 4b, 4c and 4d, transfer rollers 5a, 5b, 5c and 5d serving as transfer units, and drum cleaning devices 6a, 6b, 6c and 6d are disposed surrounding the photosensitive drums 2a, 2b, 2c and 2d, respectively. A laser exposure device 7 is disposed below spaces between the primary chargers 3a, 3b, 3c and 3d, and the development devices 4a, 4b, 4c and 4d. The development devices 4a, 4b, 4c and 4d contain yellow toner, magenta toner, cyan toner, and black toner, respectively.
The photosensitive drums 2a, 2b, 2c, and 2d are negatively-charged organic photo conductor (OPC) photosensitive members, configured of an aluminum cylinder whose outer peripheral surface is coated with an organic photo conductive member layer. The photosensitive drums 2a, 2b, 2c, and 2d are rotated at a predetermined process speed in a direction of an arrow (in a clockwise direction illustrated in FIG. 3) by a driving device (not illustrated).
The primary chargers 3a, 3b, 3c and 3d are primary charging units. The primary chargers 3a, 3b, 3c and 3d uniformly charge the surfaces of the photosensitive drums 2a, 2b, 2c, and 2d to a negative polarity by a charging bias that is applied by a charging bias power source (not illustrated).
The development devices 4a, 4b, 4c and 4d contain toner and apply the respective color toner to an electrostatic latent image formed on each of the photosensitive drums 2a, 2b, 2c, and 2d. The electrostatic latent image is thus developed (visualized) as a toner image.
The transfer rollers 5a, 5b, 5c and 5d, which are primary transfer units, are in contact with the photosensitive drums 2a, 2b, 2c and 2d via the intermediate transfer belt 8 at primary transfer portions 32a, 32b, 32c, and 32d, respectively.
The drum cleaning devices 6a, 6b, 6c and 6d include cleaning blades that remove and collect residual primary transfer toner remaining on the photosensitive drums 2a, 2b, 2c and 2d, respectively.
The intermediate transfer belt 8 extends around a secondary transfer counter roller 10, a support roller 9, and a tension roller 11. The intermediate transfer belt 8 is rotated in the direction of an arrow A (a counterclockwise direction in FIG. 3) by a drive input to the secondary transfer counter roller 10. Consequently, the surface of the intermediate transfer belt 8 facing the primary transfer portions 32a, 32b, 32c, and 32d is pulled by the secondary transfer counter roller 10, to which a driving force is input. Thus, the intermediate transfer belt 8 can stably perform primary transfer.
The intermediate transfer belt 8 is formed by a dielectric resin, such as polycarbonate, polyethylene terephthalate resin film, or polyvinylidene-fluoride resin film. The secondary transfer counter roller 10 is in contact with a secondary transfer roller 12 via the intermediate transfer belt 8. Furthermore, a belt cleaning device 13, which removes and collects residual secondary transfer toner remaining on the intermediate transfer belt 8, is disposed outside the endless intermediate transfer belt 8 and near the tension roller 11.
Moreover, a fixing device 16, which includes a fixing roller 16a and a pressure roller 16b, is arranged downstream of the secondary transfer portion 34 relative to a conveyance direction of a transfer material P and above the secondary transfer portion 34. Thus, an approximately vertical conveyance path for conveying the transfer material P is formed.
The laser exposure device 7 includes a laser emitting unit, which emits a laser beam modulated according to a time-series electric digital pixel signal of image information, which is input to the laser exposure device 7. The laser exposure device 7 also includes a polygon lens and reflection mirrors. The laser exposure device 7 exposes the photosensitive drums 2a, 2b, 2c and 2d, which are charged by the primary chargers 3a, 3b, 3c, and 3d, to form latent images of respective colors according to image information.
Image forming operation by the above-described image forming apparatus will be described below.
Upon generation of a signal to start an image forming operation, the primary chargers 3a, 3b, 3c and 3d uniformly charge the photosensitive drums 2a, 2b, 2c and 2d, which are rotated at a predetermined process speed, in the image forming units 1Y, 1M, 1C and 1Bk to a negative polarity, respectively.
A laser diode in the laser exposure device 7 emits a laser beam based on color-separated image signals that are input externally. Consequently, the emitted laser beam forms a latent image corresponding to each color on the photosensitive drums 2a, 2b, 2c and 2d via the polygon lens and reflection mirrors.
A developing bias of the same polarity as the charging polarity (i.e., negative polarity) of the photosensitive drum 2a is applied to the development device 4a. The development device 4a then applies yellow toner to the latent image formed on the photosensitive drum 2a. The electrostatic latent image is thus visualized as a toner image. A primary transfer bias, whose polarity is opposite that of the toner, i.e., a positive polarity, is applied on the transfer roller 5a. The transfer roller 5a then primarily transfers the obtained yellow toner image on the photosensitive drum 2a onto the rotating intermediate transfer belt 8 at the primary transfer portion 32a between the photosensitive drum 2a and the transfer roller 5a. The intermediate transfer belt 8, onto which the yellow toner image is transferred, is moved towards the image forming unit 1M.
In the image forming unit 1M, a magenta toner image formed on the photosensitive drum 2b similarly as the above-described yellow toner image is transferred to the intermediate transfer belt 8 at the primary transfer portion 32b. The magenta toner image is superimposed on the yellow toner image on the intermediate transfer belt 8.
Further, cyan and black toner images are formed on the photosensitive drums 2c and 2d in the image forming units 1C and 1Bk. The cyan and black toner images are then similarly sequentially superimposed on the yellow and magenta toner images transferred onto the intermediate transfer belt 8 at the primary transfer portions 32c and 32d, respectively. A full-color toner image is thus formed on the intermediate transfer belt 8.
Cleaning blades in each of the drum cleaning devices 6a, 6b, 6c and 6d remove and collect residual primary transfer toner remaining on the respective photosensitive drums 2a, 2b, 2c and 2d. 
The transfer material P is selected from a sheet cassette 17 or a manual feed tray 20. A registration roller 19 then conveys the transfer material P to the secondary transfer portion 34 between the secondary transfer counter roller 10 and the secondary transfer roller 12 via a conveyance path 18. The transfer material P is conveyed to the secondary transfer portion 34 in synchronization with timing in which a leading end of the full-color toner image on the intermediate transfer belt 8 is moved to the secondary transfer portion 34.
A secondary transfer bias, whose polarity is opposite that of the toner, i.e., a positive polarity, is applied to the secondary transfer roller 12. The secondary transfer roller 12 collectively secondarily transfers the full-color toner image onto the transfer material P.
The transfer material P, on which the full-color toner image is formed, is conveyed to the fixing device 16. The full-color toner image is heated and pressed by the fixing nip portion between the fixing roller 16a and the pressure roller 16b. Then, the toner image is heat-fused on the surface of the transfer material P. A discharge roller 21 discharges the transfer material P onto a discharge tray 22 on the upper surface of the image forming apparatus, and the series of image forming operations ends. The belt cleaning device 13 removes and collects residual secondary transfer toner remaining on the intermediate transfer belt 8.
FIG. 4 illustrates an enlarged configuration of an area around the secondary transfer portion 34. As described above, the transfer material P that is conveyed by the registration roller 19 enters the secondary transfer portion 34 through a path represented by a chain double-dashed line in FIG. 4. The intermediate transfer belt 8, carrying the full-color toner image, and the transfer material P are away from each other at an angle α just before the transfer material P reaches the secondary transfer portion 34, as illustrated in FIG. 4. Since the transfer material P rapidly comes close to the intermediate transfer belt 8 at the secondary transfer portion 34, an image defect may occur due to a discharge phenomenon. To prevent such an image defect, in recent image forming apparatuses, a driven support roller 9 is additionally disposed upstream of the secondary transfer portion 34 relative to the intermediate transfer belt 8, as illustrated in FIG. 5. A secondary transfer plane 8c is thus formed by the secondary transfer counter roller 10 and the driven support roller 9, and the transfer material P is conveyed along the secondary transfer plane 8C.
By conveying the transfer material P along the secondary transfer plane 8c, the direction of the transfer material P entering the secondary transfer portion 34 and the traveling direction of the intermediate transfer belt 8 carrying the toner image can approximately match each other. As a result, the above-described discharge phenomenon just before the secondary transfer portion 34 can be reduced, and an image defect can be prevented. In this configuration, the intermediate transfer belt 8 extends around the secondary transfer counter roller 10, the driven support roller 9, and the tension roller 11. The intermediate transfer belt 8 is rotated in the direction of the arrow A (i.e., counterclockwise direction in FIG. 5) by a drive input to the secondary transfer counter roller 10.
However, a stable rotation of the intermediate transfer belt 8 is required to meet the recent demand for high image quality. Therefore, a wrapping angle of the intermediate transfer belt 8 around the secondary transfer counter roller 10 needs to be increased in order to increase friction transmission from the secondary transfer counter roller 10 to the intermediate transfer belt 8.
However, when the driven support roller 9 is added upstream of the secondary transfer portion 34 relative to the intermediate transfer belt 8, as illustrated in FIG. 5, the wrapping angle of the intermediate transfer belt 8 around the secondary transfer counter roller 10 becomes small.
To overcome such a problem, there are two solutions as below:    1. Incline the secondary transfer plane 8c at an angle with respect to the apparatus main body from a vertical position; and    2. Increase the diameter of the secondary transfer counter roller 10.However, new problems arise in each of the above methods.
In the first method, the secondary transfer plane 8c is inclined at an angle with respect to the apparatus main body from a vertical position. Consequently, the transfer material conveyance path from the registration roller 19 to the discharge roller 21 via the secondary transfer portion 34 and the fixing device 16 is greatly inflected. In such a case, the conveyed transfer material P is discharged rightward from the secondary transfer portion 34 as illustrated in FIG. 5. The transfer material P is then bent leftward by the fixing device 16 as illustrated in FIG. 5 and conveyed to the discharge roller 21. Since the form of the transfer material P just after passing through the secondary transfer portion 34 has an effect on an unfixed toner image formed on the transfer material P, the form of the transfer material P is required to be stable.
Recently, a wide variety of transfer materials is used in an image forming apparatus, and images are formed on various transfer materials of grammage of less than 60 g/m2, up to 300 g/m2, and from thin to thick papers. If the above-described bend is too large, the form of the transfer material P just after being discharged from the secondary transfer portion 34 cannot be stably maintained. Therefore, a problem arises when the image forming apparatus accepts such various types of transfer materials.
On the other hand, if the diameter of the secondary transfer counter roller 10 is increased as in the second method, the transfer material P may twine around the secondary transfer counter roller 10. That is, as described above, the toner image on the intermediate transfer belt 8 is transferred onto the transfer material P by applying a secondary transfer bias at the secondary transfer portion 34. Consequently, the transfer material P is attracted to the intermediate transfer belt 8 by the generated electrostatic force. If the diameter of the secondary transfer roller 10 is large, it is difficult for the transfer material P to separate from the secondary transfer roller 10 owing to the stiffness of the transfer material P. As a result, the transfer material P may twine around the secondary transfer counter roller 10.
Thus, although the transferability of the transfer material P can be improved by disposing an additional roller upstream of the secondary transfer portion 34 to form a secondary transfer plane that is approximately vertical, the friction transmission from the secondary transfer counter roller 10 to the intermediate transfer belt 8 becomes insufficient.